For Space Marines and smaller, use the medium size, 4.5mm x 2.2mm, for Space Marines and larger, use the Large size 6mm x 2.2mm

For small based ships such as X-Wings, TIE Fighters, and Fang Fighters, use the Medium (4.5mm) size. For all other ships I recommend going ot the large size. For ships like the Ghost, I recommend using 2 magnets for a more secure hold.